1. Field of the Invention
The invention relates generally to the field of disc drives, and more particularly to an apparatus and method for detecting air-contamination of hydrodynamic bearings within a disc drive.
2. Description of the Related Art
Disc drives are capable of storing large amounts of digital data in a relatively small area. Disc drives store information on one or more recording media. The recording media conventionally takes the form of a circular storage disc, e.g., media, having a plurality of concentric circular recording tracks. A typical disc drive has one or more discs for storing information. This information is written to and read from the discs using read/write heads mounted on actuator arms that are moved from track to track across surfaces of the discs by an actuator mechanism.
Generally, the discs are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the discs under the read/write heads. The spindle motor generally includes a shaft supported from a base plate and a hub, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the hub interact with a stator winding to rotate the hub relative to the shaft. In order to facilitate rotation, one or more bearings are usually disposed between the hub and the shaft.
Over time, disc drive storage density has tended to increase and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage discs. For example, to achieve increased storage densities the read/write heads must be placed increasingly close to the surface of the storage disc.
From the foregoing discussion, it can be seen that the bearing assembly which supports the storage disc is of critical importance. One typical bearing assembly comprises ball bearings supported between a pair of races which allow a hub of a storage disc to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor because of low damping.
One alternative bearing design is a hydrodynamic bearing. In a hydrodynamic bearing, a lubricating fluid such as air or sleeve provides a bearing surface between a fixed member of the housing (e.g. shaft) and a rotating member of the disc hub. Typical lubricants include oil or other fluid (i.e., hydrodynamic fluid). Hydrodynamic bearings spread the bearing interface over a large surface area in comparison with a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeatable run-out.
Generally, during the manufacturing of the hydrodynamic bearings using oil or, the hydrodynamic bearing undergoes a lubricating fluid fill process. Sometimes during the lubricating fluid fill process, air is inadvertently introduced into the lubricating fluid in the form of air bubbles. Unfortunately, the air bubbles may cause fluid pressure inconsistencies within the hydrodynamic bearing. Further, during operation, the air bubbles may expand, reducing the average viscosity of the hydrodynamic bearing fluid increasing wobble or run-out between the rotating and fixed members.
Generally, for hydrodynamic bearings, such as stationary shaft and two piece hub-shaft motors, the meniscus of the fluid is checked under a vacuum for changes due to air-contamination. For example, a microscope may be used to visually check the fluid meniscus change in dimension when a vacuum is applied. If air is present in the hydrodynamic bearings, the meniscus width, height, etc., within the capillary seal or similar location at an end of the bearing may vary as a function of the amount of air present. However, the process is slow; and the construction of the bearing may make such visual inspection difficult or impossible.
Differential weight changes between reference weights may also be used to inspect for the air-contamination of fluid within a hydrodynamic bearing. Unfortunately, this methodology is time consuming and prone to inaccuracy, as the amount of air within the hydrodynamic fluid may be very small in comparison to the fluid volume. Accordingly, the measurements may lead to an increase in disc drive manufacturing time, premature disc drive failure due to inaccurate measurements, and ultimately an increase in the cost of the disc drive.
Therefore, a need exists for a method and apparatus to provide a reliable and repeatable hydrodynamic bearing air-contamination test.
Embodiments of the invention generally provide a method for detecting air-contamination of hydrodynamic bearings used with a disc drive. In one embodiment, the invention generally provides a method of measuring air-contamination within a hydrodynamic bearing of a disc drive at different levels of air pressure. The method comprises rotating a disc drive motor and providing at least one first measurement of one or more disc drive input/output signals responsive to air-contamination within the hydrodynamic bearing. The method then compares at least one second measurement of the one or more disc drive input/output signals to the at least one first measurement, the measurements being taken at two different pressures. If the at least one second measurement is different than the at least one first measurement, then determining if the difference between the first and second measurements is indicative of at least one unacceptable disc drive operational condition.
In another embodiment, the invention provides an apparatus for measuring air-contamination within a disc drive hydrodynamic bearing. The apparatus comprising a means for rotating a disc drive motor, and a means for detecting, during the rotation of the disc drive motor, at least one disc drive input/output signal responsive as a function of air-contamination within the hydrodynamic bearing between at least a first and second external pressure on the hydrodynamic bearing.